In vehicle navigation systems, a vehicle's position is determined through the accumulation of data gathered by various sensors. Typical navigation sensors include compasses to measure the absolute vehicle heading relative to the earth's magnetic field, gyroscopes and differential odometers to measure the vehicle's relative heading; and odometers to measure the absolute distance traveled by the vehicle. Errors in vehicle position result from the accumulation of measurement errors by each of the sensors.
A technique known as "map matching" has been used to correct vehicle position errors which result from the accumulation of navigation sensors' errors. Map matching uses geometrical similarities in its decision-making process. The navigation system compares the current vehicle trajectory to street geometries near the currently stored vehicle position. The system then corrects the vehicle position to the location which most closely matches the vehicle's trajectory. To accomplish this, the map matching system searches its internal map data base in the vicinity of the most recently calculated vehicle position to find street candidates which lie in the direction in which the vehicle is currently headed. The vehicle's heading, speed, and distance traveled are continuously monitored and compared to the geometry of the current "list" of street candidates. As the geometry of each street diverges from the vehicle's calculated trajectory, that street is eliminated as a possible location. This process continues until all streets are eliminated except one. That street is then stored as the current location of the vehicle.
Kao in U.S. Pat. No. 5,374,933, discloses a position correction method for a vehicle navigation system with improved accuracy over simple map matching techniques. In the preferred embodiment, the system first stores a known vehicle position in its memory. As the vehicle moves away from the known position, the system uses a variety of means to sense external signals. These means include satellite transmission receivers and magnetic compasses. When a change in the external signals occurs because of a nearby landmark, the system uses the location of the landmark to set the vehicle's position. To accomplish this, the system's data processor searches a map data base, attempting to fix the position of the sensed landmark with respect to the initial stored known position. When the system identifies the most likely landmark, the position of the vehicle is then reset to that position. This technique can be thought of as "phenomenon matching."
U.S. Pat. No. 5,345,388 issued to Kashiwazaki discloses a navigation system adapted to efficiently utilize a memory for storing locus data, thus making it possible to store more loci by lesser quantity of data, and to reference to a longer movement path by the same memory capacity. The invention also provides a navigation system adapted to preserve or store, as locus data, movement paths which were run in the past so that they can be readily referenced to when one goes to the same destination for a second time, thus permitting support of driving to be effective. Storing of locus data is carried out by detecting a traveling azimuth to judge a change in the travelling azimuth to compress locus data only to data in a predetermined range before and after the position where the azimuth is changed.
There are several well known in the art navigation systems incorporating satellite positioning system (SATPS) for navigating various mobile objects such as automobiles, airplanes, ships, or the like in a global geometrical region. Typically, such a navigation system on a mobile object has a SATPS receiver which receives radio waves transmitted from three or more SATPS satellites, determines the position of a reception point where its radio-wave receiver has received the radio waves, based on quasi-distance data between the SATPS satellites and the reception point, including a time offset of the receiver, and positional data of the SATPS satellites, and outputs the data about the determined position. Since, however, the radio waves from the SATPS satellites may not necessarily be received under good conditions because of receiving environments and weather conditions, some navigation systems which incorporate a SATPS receiver are also combined with a self-operating sensor which produces the positional data of its own. In operation, the navigation system selects whichever of the data from the SATPS receiver and the self-operating sensor is more accurate at the time for higher positioning accuracy for better navigation. The self-operating sensor for use in automobile navigation systems may comprise an orientation sensor such as a geomagnetic sensor or a gas-rate gyroscope, or a speed sensor such as a sensor for detecting the rotational speed of a crankshaft. The navigation system may employ a PDOP (Position Dilution Of Precision) value for determining the accuracy with which the SATPS receiver determines the position. If the PDOP value is equal to or lower than a predetermined value, then the navigation system selects the data from the SATPS receiver for navigation. The PDOP value is used in a three-dimensional positioning system in which the three-dimensional position of a reception point is determined by simultaneously measuring the distances up to four or more SATPS satellites. The PDOP value is representative of how positional errors of the SATPS satellites are reflected by the calculated present position of the reception point. If the PDOP value is larger, then it indicates that the calculated present position of the reception point is suffering a greater error. When only a two-dimensional positioning process is available, the PDOP value cannot be obtained and the navigation system cannot detect a reduction in the positional accuracy owing to an error caused by a change in the altitude of the reception point. In the event of an intentional accuracy reduction known as selective availability (SA), the effective position accuracy (related to the PDOP value) is known to be larger than the basic SATPS accuracy, which results in a lower degree of positional accuracy. Thus, the navigation system can automatically select the data from the self-operating sensor even when the positional accuracy of the data from the self-operating sensor is actually lower than the positional accuracy of the data from the SATPS receiver.
Fukushima in U.S. Pat. No. 5,293,318, discloses a navigation system including a global positioning system (GPS) receiver and a self-operating sensor for determining the position of a reception point or a mobile object such as an automobile in a global geometrical region with the increased accuracy when the PDOP value cannot be determined with an adequate degree of precision. This goal is achieved by providing a navigation system with a first positional data produced by the GPS receiver and a second positional data produced by a self-operating sensor. A data processor compares previous and present first positional data to determine whether the difference therebetween is equal to or smaller than a first predetermined value, and also determines whether a PDOP value of the present first positional data is equal to or smaller than a second predetermined value. The present first positional data and the second positional data are compared to determine whether the difference between the present position of the automobile as indicated by the present first positional data and the present position of the automobile as indicated by the second present positional data is equal to or greater than a third predetermined value. The data processor outputs the first positional data as present positional data if the difference between the previous and present first positional data is equal to or smaller than the first predetermined value, if the PDOP value is equal to or smaller than the second predetermined value, and also if the difference between the present position of the automobile as indicated by the present first positional data and the present position of the automobile as indicated by the second present positional data is equal to or greater than the third predetermined value.
Kato in U.S. Pat. No. 5,272,483, discloses another navigation system which is also intended to overcome the problems of the inadequate determination of the PDOP when a positioning mode switches between two- and three-dimensional positioning modes. The navigation system includes a GPS receiver and a self-operating sensor for navigating a reception point or a mobile object such as an automobile accurately at all times in a global geometrical region and displaying the position of the mobile object accurately on a display unit based on effective use of GPS data from the GPS receiver.
A relative positioning system (RPS), such as a dead reckoning system with map matching, is disclosed in U.S. Pat. No. 4,796,191 issued to Honey et al. The RPS is different from an absolute positioning system (APS) like a Global Positioning System (GPS) because the RPS can operate in a fully self-contained way, requiring no equipment outside the vehicle in which it is used. It typically has high accuracy over significant intervals of time. It is linked to an electronic map of roads which can automatically eliminate minor vehicle position errors and measurement noise and provide a graphical user display. For example, as a vehicle using such a system moves on board wheel sensors, a magnetic compass and/or other sensing means computes the vehicle's position using dead reckoning techniques. The computed position is compared frequently with an electronically stored map of roads. If the computed position does not correspond to a location on the nearest appropriate road, the system automatically corrects the vehicle's position to place it on that nearest road. However, the dead-reckoning system has a number of disadvantages. One of the disadvantages is that sometimes navigation performance can degrade if the map matching relocates the vehicle's position to an incorrect road. This can occur because of an extreme anomalous magnetic field, wheel slippage or map errors. Another disadvantage arises if the difference between the computed vehicle position and the nearest appropriate road is too large, i.e. exceeds a predetermined allowable error estimate. Under these circumstances the dead reckoning system will not update its position.
Mathis et al. in U.S. Pat. No. 5,311,195, discloses a combined relative and absolute positioning method and apparatus which overcomes the disadvantages of the pure RPS. The absolute positioning system (APS) can include a Loran-C or a GPS. The information provided by the APS is used for updating RPS position, so that when the RPS is reset or updated, it is updated to the current APS position with its offset.
In U.S. Pat. No. 5,283,575 issued to Kao and Huang, a vehicle navigation system employing a fuzzy logic is disclosed. The sensors continually feed position coordinates to a processing unit that traces the vehicle's path in a road database. The errors of positioning sensors and routing computers are overcome by using the fuzzy logic. Fuzzy logic-based reasoning is used to determine the most probable location of the vehicle, whether off- or on-road, thereby correcting errors in its raw path as sensed or computed.
Thus, according to the prior art, for a customer to display his position on a particular road on some map, he has to translate the "raw" fixer obtained by using some positioning system to the position on the particular road. All companies who provide the position determining means do this step of translating the raw "fixer" to the position on the particular road after the raw fixer is obtained. Different companies use different maps, data timers, displays, etc.
What is needed is to add value to the SATPS receiver, so that the SATPS receiver would have the built-in map-matching capability by translating inside the receiver the raw fixer into the position on a particular road, wherein the receiver's position could be defined in terms of a uni-dimensional coordinate system.